Lubricating oil composition

ABSTRACT

The present invention provides a lubricating oil composition improved in fuel-saving properties by reducing lubrication resistance within the same viscosity grade oils. The lubricating oil composition comprises a lubricating base oil and a viscosity index improver with a PSSI of 30 or lower blended such that the viscosity index of the composition is 160 or greater and the 100° C. kinematic viscosity loss after shearing is 15 percent or less.

FIELD OF THE INVENTION

The present invention relates to lubricating oil compositions, more specifically to those improved in fuel-saving properties by reducing lubrication resistance. Further more specifically, the present invention relates to lubricating oils of SAE xW-30 grade for internal combustion engines, particularly suitable for diesel engines, which oils can be enhanced in fuel-saving properties more by reducing lubrication resistance than those of the same viscosity grade.

BACKGROUND ART

Engine oils have been demanded to be improved in fuel-saving properties in view of environmental issues such as reduction in carbon dioxide gas emission. Therefore, blend of friction modifiers such as MoDTC or polymeric viscosity index improvers (as disclosed in Patent Document 1 below) or lowering the viscosity or increasing the viscosity index of lubricating oil has been carried out. Since a friction modifier such as MoDTC is extremely prevented from exhibiting its friction reduction effect when used in a diesel engine oil in which much soot contaminates and thus poor in sustainability of its fuel-saving properties, it is now important to formulate a lubricating oil composition basically not relying on the use of a friction modifier, i.e., to lower the viscosity or increase the viscosity index of a lubricating oil.

However, it is difficult to lower the viscosity of a lubricating oil for a diesel engine oil used under sever sliding conditions to the viscosity level as achieved with an SAE xW-20 grade oil because such a lower viscosity causes increased wear at sliding parts, accelerated deterioration of the oil by increased evaporation loss and aggravated exhaust gas properties (see, for example, Patent Document 2 below). As the result, for a diesel engine oil, improvements have been focused on those of SAE (xW-) 30 viscosity grade or higher.

In order to improve the fuel-saving properties of a lubricating oil composition, it is most common to lower the viscosity of the base oil and increase the viscosity index of the composition and most effective to blend a polymethacrylate viscosity index improver with a higher molecular weight in a somewhat large amount in anticipation of a reduction in kinematic viscosity after shearing. However, because the kinematic viscosity loss of the composition of this case is in the order of 20 percent or higher, it is necessary to set the kinematic viscosity of the composition to a higher level in advance so as to retain the same viscosity grade after shearing. There is a limit in this technique to improve the fuel-saving properties of an engine oil throughout the initial use to a long term use.

(1) Japanese Patent Application Laid-Open Publication No. 8-302378

(2) Japanese Patent Application Laid-Open Publication No. 2001-181664

DISCLOSURE OF THE INVENTION

In view of the foregoing circumstances, the present invention has an object to provide a lubricating oil composition that can enhance the fuel-saving properties of a lubricating oil more than those of the same viscosity grade by reducing the lubrication resistance of, for example, a lubricating oil of SAE xW-30 grade for a internal combustion engine, in particular a diesel engine.

As the result of the extensive research and study of the inventors, the present invention was accomplished on the basis of the finding that the aforesaid object was able to be achieved by blending a viscosity index improver so that among lubricating oils of the same viscosity grade, the viscosity index thereof is enhanced to a certain level or higher and the viscosity loss after shearing is reduced to a certain level or lower.

That is, the present invention relates to a lubricating oil composition comprising a lubricating base oil and a viscosity index improver with a PSSI of 30 or lower blended such that the viscosity index of the composition is 160 or greater and the 100° C. kinematic viscosity loss after shearing is 15 percent or less.

The present invention also relates to the foregoing lubricating oil composition wherein the viscosity index improver is a styrene-diene copolymer viscosity index improver with a PSSI of 30 or lower.

The present invention also relates to the foregoing lubricating oil composition wherein the viscosity index improver is a polymethacrylate viscosity index improver with a PSSI of 15 or lower.

The present invention also relates to the foregoing lubricating oil composition wherein the 100° C. TBS viscosity of the composition is 7 mPa·s or lower.

The present invention also relates to the foregoing lubricating oil composition wherein the 100° C. kinematic viscosity of the composition is from 9.3 to 11.0 mm²/s, the 40° C. kinematic viscosity is from 40 to 60 mm²/s and the 100° C. kinematic viscosity after shearing is 9.3 mm²/s or higher.

The present invention also relates to the foregoing lubricating oil composition further comprising a metal salt of a phosphorus-containing acid ester.

The present invention also relates to the foregoing lubricating oil composition further comprising a metallic detergent, an ashless dispersant, and an anti-oxidant.

The present invention also relates to the foregoing lubricating oil composition wherein the sulfated ash content is from 0.1 to 2 percent by mass.

EFFECTS OF THE INVENTION

The lubricating oil composition of the present invention is a lubricating oil for internal combustion engines that can be enhanced in fuel-saving properties more by reducing lubrication resistance, than those of the same viscosity grade.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be described in more detail below.

A lubricating base oil used for the lubricating oil composition of the present invention may be a mineral base oil and/or a synthetic base oil which have been used in conventional lubricating oils.

Examples of the mineral base oil include those which can be produced by subjecting a lubricating oil fraction produced by vacuum-distilling a topped crude resulting from atmospheric distillation of a crude oil, to any one or more treatments selected from solvent deasphalting, solvent extraction, hydrocracking, solvent dewaxing, catalytic dewaxing, hydrorefining, sulfuric acid treatment, and clay treatment; and wax-cracked/isomerized mineral oils produced by hydrocracking and/or isomerizing a raw material containing wax the main component of which is n-paraffin such as slack wax and GTL WAX (Gas to Liquid Wax) produced through a Fischer-Tropsch process.

Examples of the synthetic base oil include poly-α-olefins such as 1-octene oligomer, 1-decene oligomer and ethylene-propylene oligomer, and hydrogenated compounds thereof; isobutene oligomers and hydrogenated compounds thereof; isoparaffins; alkylbenzenes; alkylnaphthalenes; diesters such as ditridecyl glutarate, dioctyl adipate, diisodecyl adipate, ditridecyl adipate and dioctyl cebacate; polyol esters (trimethylolpropane esters such as trimethylolpropane caprylate, trimethylolpropane pelargonate and trimethylolpropane isostearate and pentaerythritol esters such as pentaerythritol 2-ethylhexanoate and pentaerythritol pelargonate); polyoxyalkylene glycols; dialkyldiphenyl ethers; and polyphenyl ethers.

In the present invention, the lubricating base oil may be a mineral base oil, a synthetic base oil or a mixture of any two or more lubricating oils selected from these base oils. For example, the base oil used in the present invention may be one or more of the mineral base oils or synthetic base oils or a mixed oil of one or more of the mineral base oils and one or more of the synthetic base oils.

There is no particular restriction on the viscosity index of the lubricating base oil. However, the viscosity index is preferably 90 or higher, more preferably 115 or higher, more preferably 120 or higher. The use of a base oil with a higher viscosity index enables the production of a composition with higher oxidation stability and excellent fuel-saving properties and low temperature viscosity characteristics. The viscosity index of the lubricating base oil is usually 250 or lower, preferably 200 or lower. For a mineral lubricating base oil, the viscosity index is desirously 160 or lower because it is excellent in availability, production cost and low temperature viscosity characteristics.

There is no particular restriction on the NOACK evaporation loss of the lubricating base oil. However, the NOACK evaporation loss is usually 20 percent by mass or less, preferably 16 percent by mass, more preferably from 10 to 15 percent by mass. If the base oil has a NOACK evaporation loss of more than 20 percent by mass, the fuel-saving properties are unlikely to be maintained for a long period of time because the amount of the base oil to be lost by evaporation would tend to be larger and thus an increase in viscosity and the condensation of additives likely occur. If the base oil has a NOACK evaporation loss of less than 10 percent, the fuel-saying properties of the resulting composition is unlikely to be enhanced.

There is no particular restriction on the 100° C. kinematic viscosity of the lubricating base oil. However, the 100° C. kinematic viscosity is preferably from 1 to 20 mm²/s, more preferably from 3.5 to 6 mm²/s, more preferably from 3.7 to 4.5 mm²/s, most preferably from 3.8 to 4.2 mm²/s. If the 100° C. kinematic viscosity is lower than 1 mm²/s, the amount of the base oil to be lost by evaporation would be large due to the heat generated from an internal combustion engine, and the viscosity of the resulting composition would possibly increase or exhaust gas would be adversely affected. On the other hand, if the 100° C. kinematic viscosity exceeds 20 mm²/s, the resulting composition would cause an increase in power loss due to viscous resistance and thus unlikely exhibits the fuel-saving properties to the full extent.

There is no particular restriction on the 40° C. kinematic viscosity of the lubricating base oil. However, the 40° C. kinematic viscosity is preferably from 10 to 100 mm²/s, more preferably from 13 to 25 mm²/s, more preferably from 15 to 20 mm²/s, particularly preferably from 16 to 19 mm²/s. The use of a base oil with a 40° C. kinematic viscosity of 100 mm²/s or lower enables the resulting composition to be enhanced in viscosity index and to exhibit excellent fuel-saving properties. On the other hand, the 40° C. kinematic viscosity is preferably 10 mm²/s or higher with the objective of preventing wear and suppressing evaporation loss.

There is no particular restriction on the % C_(P) of the lubricating base oil. However, the % C_(P) is preferably 60 or greater, more preferably 70 or greater, more preferably 80 or greater because the resulting composition will be more excellent in an effect of inhibiting increases in the acid number and viscosity in the presence of NOx. There is no particular restriction on the upper limit of the % Cp. As one embodiment of the present invention, the % C_(P) may be from 95 to 100. However, the % C_(P) is preferably 95 or less in view of more excellent soot dispersibility and sludge dissolubility.

There is no particular restriction on the % C_(N) of the lubricating base oil. The % C_(N) is preferably 40 or less, more preferably 30 or less, more preferably 20 or less, particularly preferably 15 or less because the resulting composition will be more excellent in an effect of inhibiting increases in the acid number and viscosity in the presence of NOx.

There is no particular restriction on the % C_(A) of the lubricating base oil. However, the % C_(A) is preferably 5 or less, more preferably 2 or less, more preferably 1 or less, more preferably 0.5 or less, because the resulting composition will be more excellent in an effect of inhibiting increases in the acid number and viscosity in the presence of NOx.

There is no particular restriction on the % C_(P)% C_(N) of the lubricating base oil. The % C_(P)% C_(N) is preferably 1 or greater, more preferably 3 or greater, more preferably 5 or greater, particularly preferably 6 or greater because the resulting composition will be more excellent in an effect of inhibiting increases in the acid number and viscosity in the presence of NOx. There is no particular restriction on the upper limit of the % C_(P)% C_(N). However, the % C_(P)% C_(N) is preferably 20 or less, more preferably 15 or less, more preferably 10 or less, particularly preferably 8 or less in view of more excellent soot dispersibility and sludge dissolubility.

The % C_(A), % C_(P), and % C_(N) used herein denote the percentages of the aromatic carbon number in the total carbon number, the paraffin carbon number in the total carbon number, and the naphthene carbon number in the total carbon number, respectively, determined by a method (n-d-M ring analysis) in accordance with ASTM D 3238-85.

There is no particular restriction on the sulfur content of the lubricating base oil. However, the sulfur content is preferably 0.3 percent by mass or less, more preferably 0.2 percent by mass or less, more preferably 0.1 percent by mass or less, most preferably 0.01 percent by mass or less.

In the present invention, the lubricating base oil is preferably a mineral base oil and/or a poly-α-olefin base oil. The mineral base oil is particularly preferably a hydrocracked mineral oil or a wax isomerized mineral oil. The poly-α-olefin base oil is preferably a copolymer of an α-olefin having 6 to 18 carbon atoms, particularly 6 to 12 carbon atoms or a hydrogenated compound thereof.

Examples of Component (A) used in the present invention, i.e., viscosity index improver include non-dispersant type and dispersant type viscosity index improvers. Specific examples include non-dispersant type and dispersant type polymethacrylates, dispersant type ethylene-α-olefin copolymers and hydrogenated compounds thereof, polyisobutylenes and hydrogenated compounds thereof, styrene-diene copolymers, styrene-maleic anhydride ester copolymers, and polyalkylstyrenes. Among these viscosity index improvers, it is preferable to use non-dispersant type and/or dispersant type viscosity index improvers, most preferably dispersant type viscosity index improvers having a weight average molecular weight of preferably 10,000 or greater, more preferably 20,000 or greater, more preferably 50,000 or greater, particularly preferably 80,000 or greater, and usually 1,000,000 or less, preferably 400,000 or less, more preferably 300,000 or less, more preferably 200,000 or less, particularly preferably 150,000 or less.

Specific examples of the non-dispersant type viscosity index improver include homopolymers of monomers selected from the group consisting of compounds represented by formulas (1), (2) and (3) below (hereinafter referred to as “monomer (M-1)”), copolymers of two or more of monomers (M-1), and hydrogenated compounds thereof.

Specific examples of the dispersant type viscosity index improver include copolymers of two or more of monomers selected from the group consisting of compounds represented by formulas (4) and (5) below (hereinafter referred to as “monomer (M-2)”) and hydrogenated compounds thereof; and copolymers of one or more of monomers (M-1) selected from the group consisting of compounds represented by formulas (1), (2) and (3) below with one or more of monomers (M-2) selected from the group consisting of compounds represented by formulas (4) and (5) below and hydrogenated compounds thereof.

In formula (1), R¹ is hydrogen or methyl, and R² is hydrogen or an alkyl group having 1 to 18 carbon atoms.

Specific examples of the alkyl group having 1 to 18 carbon atoms for R² include those, which may be straight-chain or branched, such as methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl and octadecyl groups.

In formula (2), R³ is hydrogen or methyl, and R⁴ is hydrogen or a hydrocarbon group having 1 to 12 carbon atoms.

Specific examples of hydrocarbon groups having 1 to 12 carbon atoms for R⁴ include alkyl groups, which may be straight-chain or branched, such as methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl and dodecyl groups; cycloalkyl groups having 5 to 7 carbon atoms, such as cyclopentyl, cyclohexyl and cycloheptyl groups; alkylcycloalkyl groups, of which the alkyl groups may bond to any position of the cycloalkyl group, having 6 to 11 carbon atoms, such as methylcyclopentyl, dimethylcyclopentyl, methylethylcyclopentyl, diethylcyclopentyl, methylcyclohexyl, dimethylcyclohexyl, methylethylcyclohexyl, diethylcyclohexyl, methylcycloheptyl, dimethylcycloheptyl, methylethylcycloheptyl and diethylcycloheptyl groups; alkenyl groups, which may be straight-chain or branched and the position of which the double bond may vary, such as vinyl (ethenyl), propenyl, butenyl, pentenyl, hexenyl, heptenyl, octenyl, nonenyl, decenyl, undecenyl and dodecenyl groups; aryl groups such as phenyl and naphtyl groups; alkylaryl groups, of which the alkyl groups may be straight-chain or branched and bond to any position of the aryl group, having 7 to 12 carbon groups, such as tolyl, xylyl, ethylphenyl, propylphenyl, butylphenyl, pentylphenyl and hexylphenyl groups; and arylalkyl groups, of which the alkyl groups may be straight-chain or branched, having 7 to 12 carbon atoms, such as benzyl, phenylethyl, phenylpropyl, phenylbutyl, phenylpentyl and phenylhexyl groups.

In formula (3), Z¹ and Z² are each independently hydrogen, an alkoxy group having 1 to 18 carbon atoms represented by formula —OR⁵ wherein R⁵ is an alkyl group having 1 to 18 carbon atoms, or a monoalkylamino group having 1 to 18 carbon atoms represented by formula —NHR⁶ wherein R⁶ is an alkyl group having 1 to 18 carbon atoms.

In formula (4) above, R⁷ is hydrogen or methyl, R⁸ is an alkylene group having 1 to 18 carbon atoms, E¹ is an amine residue or heterocyclic residue having 1 or 2 nitrogens and 0 to 2 oxygens, and a is an integer of 0 or 1.

Specific examples of alkylene groups having 1 to 18 carbon atoms for R⁸ include ethylene, propylene, butylene, pentylene, hexylene, heptylene, octylene, nonylene, decylene, undecylene, dodecylene, tridecylene, tetradecylene, pentadecylene, hexadecylene, heptadecylene and octadecylene groups, all of which may be straight-chain or branched.

Specific examples of groups represented by E¹ include dimethylamino, diethylamino, dipropylamino, dibutylamino, anilino, toluidino, xylidino, acetylamino, benzoilamino, morpholino, pyrrolyl, pyrrolino, pyridyl, methylpyridyl, pyrrolidinyl, piperidinyl, quinonyl, pyrrolidonyl, pyrrolidono, imidazolino and pyrazino groups.

In formula (5), R⁹ is hydrogen or methyl, and E² is an amine residue or heterocyclic residue having 1 or 2 nitrogens and 0 to 2 oxygens.

Specific examples of groups represented by E² include dimethylamino, diethylamino, dipropylamino, dibutylamino, anilino, toluidino, xylidino, acetylamino, benzoilamino, morpholino, pyrrolyl, pyrrolino, pyridyl, methylpyridyl, pyrrolidinyl, piperidinyl, quinonyl, pyrrolidonyl, pyrrolidono, imidazolino and pyrazino groups.

Preferable examples of monomers (M-1) include alkylacrylates having 1 to 18 carbon atoms; alkylmethacrylates having 1 to 18 carbon atoms; olefins styrene, methylstyrene, maleic anhydride ester and maleic anhydride amide, each having 2 to 20 carbon atoms, and mixtures thereof.

Preferred examples of monomers (M-2) include dimethylaminomethylmethacrylate, diethylaminomethylmethacrylate, dimethylaminoethylmethacrylate, diethylaminoethylmethacrylate, 2-methyl-5-vinylpyridine, morpholinomethylmethacrylate, morpholinoethylmethacrylate, N-vinylpyrrolidone, and mixtures thereof.

There is no particular restriction on the copolymerization molar ratio of a copolymer of monomers (M-1) and (M-2). However, preferably, monomer (M-1):monomer (M-2)=80:20 to 95:5. Any copolymerization method may be used. For example, such copolymers are generally produced with ease by radical-solution polymerization of monomers (M-1) with monomers (M-2) in the presence of a polymerization initiator such as benzoyl peroxide.

The PSSI (permanent shear stability index) of the viscosity index improver used in the present invention is necessarily 30 or lower, preferably 20 or lower, more preferably 15 or lower, more preferably 12 or lower. On the other hand, the PSSI is preferably 3 or greater, more preferably 5 or greater because the viscosity index improver would be less effective in enhancing the viscosity index and fuel-saving properties of the resulting lubricating oil composition if the PSSI is too low.

The term “PSSI” used herein denotes the permanent shear stability index of a polymer calculated on the basis of the data measured with ASTM D 6278-02 (Test Method for Shear Stability of Polymer Containing Fluids Using a European Diesel Injector Apparatus) in conformity with ASTM D 6022-01 (Standard Practice for Calculation of Permanent Shear Stability Index).

The content of the aforesaid viscosity index improver in the lubricating oil composition is usually from 1 to 30 percent by mass, preferably from 5 to 25 percent by mass, more preferably from 8 to 20 percent by mass, particularly preferably from 12 to 18 percent by mass on the basis of the total mass of the composition.

In the present invention, the viscosity index improver with a PSSI of 30 or less is blended in such an amount that the viscosity index of the composition is 160 or greater, preferably 170 or greater, more preferably 180 or greater. There is no particular restriction on the upper limit of the viscosity index. However, the upper limit is usually 300 or lower, preferably 250 or lower, more preferably 200 or lower, more preferably 190 or lower. A lubricating oil composition can be lowered in viscosity in the practical temperature region and thus enhanced in fuel-saving properties by blending a viscosity index improver in such an amount that the viscosity index of the composition is 160 or greater. In the present invention, in the case of enhancing the viscosity index of the composition, it is preferable to ensure that the amount of a viscosity index improver to be blended and the kinematic viscosity of the composition do not become too high. It is particularly preferable to blend a viscosity index improver in such an amount that the viscosity index of the composition is 200 or lower. As the result, a composition that is also excellent in high temperature detergency can be produced.

In the present invention, a viscosity index improver with a PSSI of 30 or lower is blended such that the 100° C. kinematic viscosity loss after shearing is 15 percent or less. The term “100° C. kinematic viscosity loss after shearing” denotes the data ((V1−V2))/V1×100) (%) calculated on the basis of the 100° C. kinematic viscosity measured after a shear test (V2) and the 100° C. kinematic viscosity measured before the shear test (V1) in accordance with ASTM D 6278-02 (Test Method for Shear Stability of Polymer Containing Fluids Using a European Diesel Injector Apparatus).

The viscosity index improver used in the present invention is preferably a polymethacrylate viscosity improver because it can enhance the viscosity index and is excellent in an effect of improving the fuel-saving properties of the composition. The PSSI of the polymethacrylate viscosity index improver used in the present invention is 30 or lower, preferably 15 or lower, more preferably 10 or lower, particularly preferably 8 or lower and preferably 1 or greater, more preferably 3 or greater. In particular, it is preferable to use a polymethacrylate viscosity index improver with a PSSI of 15 or lower.

It is also preferable to use a styrene-diene copolymer viscosity index improver because it can reduce the practical viscosity under high temperature shear conditions and is excellent in fuel-saving properties and more excellent in high temperature detergency. The PSSI of the styrene-diene copolymer viscosity index improver used in the present invention is 30 or lower, preferably 15 or lower, more preferably 12 or lower and preferably 1 or greater, more preferably 3 or greater, more preferably 5 or greater, particularly preferably 8 or greater. Examples of the styrene-diene copolymer viscosity index improver include copolymers and hydrogenated compounds thereof, of one or more types of styrene monomers or (co) polymers selected from styrene, polystyrenes and hydrogenated compounds thereof and one or more types of diene monomers or (co)polymers selected from dienes, polydienes and hydrogenated compounds thereof. The diene monomers or (co)polymers are preferably selected from butadiene monomers or (co)polymers such as butadiene, polybutadiene and hydrogenated compounds thereof and isoprene monomers or (co)polymers such as isoprene, polyisoprenes and hydrogenated compounds thereof, more preferably selected from the isoprene monomers or (co)polymers. These styrene-diene copolymer viscosity index improvers may be alternating, random or block copolymers of styrene monomers or (co)polymers and diene monomers or (co)polymers. Amongst, these viscosity index improvers are more preferably block copolymers of styrene monomers or (co)polymers and dine monomers or (co)polymers, more preferably block copolymers of styrene monomers or (co)polymers and diene monomers (co)polymers, particularly preferably block copolymers having polystyrene blocks and hydrogenated polyisoprene blocks.

Examples of such block copolymers having polystyrene blocks and hydrogenated polyisoprene blocks include block copolymers having a structure represented by EP-S (wherein EP is a hydrogenated polyisoprene block having an number average molecular weight (Mn) prior to hydrogenation of 20,000 to 150,000 and S is a polystyrene block having a number average molecular weight (Mn) of 10,000 to 60,000. These block copolymers are particularly preferably used in the present invention.

Other examples of the block copolymers having polystyrene blocks and hydrogenated polyisoprene blocks include asymmetric triblock copolymers having a structure represented by EP′-S-EP′ (wherein EP′ is a first hydrogenated polyisoprene block having a number average molecular weight (Mn) prior to hydrogenation of 40,000 to 150,000, S is a polystyrene block having a number average molecular weight (Mn) of 25,000 to 60,000, EP″ is a second hydrogenated polyisoprene block having a number average molecular weight (Mn) prior to hydrogenation of 2,5000 to 30,000, and the molecular weight ratio EP′/EP″ is at least 4).

Other examples include star polymers having a structure represented by (EP′-S-EP″)_(n)-X (wherein EP′ is a hydrogenated block of a first polyisoprene (I′) having a number average molecular weight (Mn) prior to hydrogenation of 10,000 to 100,000, S is a polystyrene block having a number average molecular weight (Mn) of 6,000 to 50,000, EP″ is a hydrogenated block of a second polyisoprene (I″) having a number average molecular weight (Mn) prior to hydrogenation of 2,500 to 50,000, the molecular weight ratio (Mw) of I′/I″ is at least 1.4, X is a core configured by a polyalkenyl coupling agent, and n is an average arm number per star molecule formed by reacting 2 moles or more of a polyalkenyl coupling agent per mole of (EP′-S-EP″) arm.

The lubricating oil composition of the present invention preferably contains (B) a phosphorus-containing compound.

There is no particular restriction on the phosphorus-containing compound as long as it contains phosphorus in its molecules. However, the phosphorus-containing compound is preferably at least one type of compound selected from the group consisting of phosphorus compounds represented by formulas (6) and (7) below, metal and amine salts thereof, and derivatives thereof:

wherein X¹, X², and X³ are each independently oxygen or sulfur, and R¹⁰, R¹¹, and R¹² are each independently hydrogen, a hydrocarbon group having 1 to 30 carbon atoms or a substitute containing a hetero element such as N, S, O having 1 to 30 carbon atoms

wherein X⁴, X⁵, X⁶, and X⁷ are each independently oxygen or sulfur, (or one or two of X⁴, X⁵ and X⁶ may be a single bond or a (poly)oxyalkylene group), and R¹³, R¹⁴, and R¹⁵ are each independently hydrogen, a hydrocarbon group having 1 to 30 carbon atoms, or a substitute containing a hetero element such as N, S, O having 1 to 30 carbon atoms.

Examples of the hydrocarbon groups having 1 to 30 carbon atoms for R¹⁰ to R¹⁵ include alkyl, cycloalkyl, alkenyl, alkyl-substituted cycloalkyl, aryl, alkyl-substituted aryl, and arylalkyl groups. The hydrocarbon groups are preferably alkyl groups having 1 to 30 carbon atoms and aryl groups having 6 to 24 carbon atoms, more preferably alkyl groups having 3 to 18 carbon atoms, more preferably alkyl groups having 4 to 12 carbon atoms. These hydrocarbon groups may contain oxygen, nitrogen, or sulfur in their molecules but desirably contain carbon and hydrogen only.

Examples of phosphorus compounds represented by formula (6) include phosphorous acid; monothiophosphorous acid; dithiophosphorous acid; trithiophosphorous acid; phosphorous acid monoesters, monothiophosphorous acid monoesters, dithiophosphorous acid monoesters, and trithiophosphorous acid monoesters, each having any one of the above-described hydrocarbon groups having 1 to 30 carbon atoms; phosphorous acid diesters, monothiophosphorous acid diesters, dithiophosphorous acid diesters, and trithiophosphorous acid diesters, each having any two of the above-described hydrocarbon groups having 1 to 30 carbon atoms; phosphorous acid triesters, monothiophosphorous acid triesters, dithiophosphorous acid triesters, and trithiophosphorous acid triesters, each having any three of the above-described hydrocarbon groups having 1 to 30 carbon atoms; and mixtures thereof.

Examples of phosphorus compounds represented by formula (7) include phosphoric acid; monothiophosphoric acid; dithiophosphoric acid; trithiophosphoric acid; tetrathiophosphoric acid; phosphoric acid monoesters, monothiophosphoric acid monoesters, dithiophosphoric acid monoesters, trithiophosphoric acid monoesters, and tetrathiophosphoric acid monoesters, each having any one of the above-described hydrocarbon groups having 1 to 30 carbon atoms; phosphoric acid diesters, monothiophosphoric acid diesters, dithiophosphoric acid diesters, trithiophosphoric acid diesters, and tetrathiophosphoric acid diesters, each having any two of the above-described hydrocarbon groups having 1 to 30 carbon atoms; phosphoric acid triesters, monothiophosphoric acid triesters, dithiophosphoric acid triesters, trithiophosphoric acid triesters, and tetrathiophosphoric acid triesters, each having any three of the above-described hydrocarbon groups having 1 to 30 carbon atoms; phosphonic acid, phosphonic acid monoesters, and phosphonic acid diesters, each having any one to three of the above-described hydrocarbon groups having 1 to 30 carbon atoms; the phosphoric acid compounds exemplified above but having a (poly)oxyalkylene group having 1 to 4 carbon atoms; derivatives of the phosphorus compounds exemplified above, such as β-dithiophosphorylated propionic acid and reaction products of dithiophosphates and olefin cyclopentadiene or (methyl)methacrylates; and mixtures thereof.

Examples of the salts of phosphorus compounds represented by formulas (6) and (7) include salts produced by allowing a metal base such as a metal oxide, a metal hydroxide, a metal carbonate and a metal chloride or a nitrogen-containing compound such as ammonia and an amine compound having in its molecules only a hydrocarbon group having 1 to 30 carbon atoms or a hydroxyl group-containing hydrocarbon group having 1 to 30 carbon atoms to react with a phosphorus compound and neutralize the whole or part of the remaining acid hydrogen.

Specific examples of the metals of the above-mentioned metal bases include alkali metals such as lithium, sodium, potassium, and cesium, alkaline earth metals such as calcium, magnesium, and barium, and heavy metals such as zinc, copper, iron, lead, nickel, silver, manganese, and molybdenum. Among these metals, preferred are alkaline earth metals such as magnesium and calcium, and zinc.

Specific examples of the nitrogen-containing compound include ammonia, monoamines, diamines, and polyamines.

Among these nitrogen-containing compounds, preferable examples include aliphatic amines having an alkyl or alkenyl group having 10 to 20 carbon atoms, which may be straight-chain or branched, such as decylamine, dodecylamine, dimethyldodecylamine, tridecylamine, heptadecylamine, octadecylamine, oleylamine, and stearyl amine.

The lubricating oil composition of the present invention particularly desirously contains a phosphorus-containing compound containing at least one type selected from the following (B1) to (B3) as the main component:

(B1) zinc dialkyldithiophosphates having a secondary alkyl group selected from those having 3 to 8 carbon atoms; (B2) zinc dialkyldithiophosphates having a primary alkyl group selected from those having 3 to 8 carbon atoms; and (B3) metal salts of sulfur-free phosphorus-containing acids.

Examples of Components (B1) and (B2) include those represented by formula (8):

In formula (8), R¹⁶, R¹⁷, R¹⁸, and R¹⁹ may be the same or different from each other and are each independently a secondary or primary alkyl group having 3 to 8 carbon atoms, preferably a secondary alkyl group having 3 to 6 carbon atoms or a primary alkyl group having 6 to 8 carbon atoms and may have in the same molecule alkyl group of different carbon number or different structure (secondary, primary).

In the present invention, the lubricating oil composition contains preferably Component (B1) because it is likely to inhibit wear caused by soot contaminated in the composition even though the concentration of Component (B1) is low, also preferably Component (B2) because it can enhance oxidation stability and base number retention properties significantly, and also most preferably Components (B1) and (B2) in combination because they can enhance the effect of inhibiting wear caused by soot contaminated in the composition and base number retention properties at a higher level in a well-balanced manner.

Component (B3) is a metal salt of a sulfur-free phosphorus-containing acid. Typical examples include metal salts of phosphorus compounds represented by formula (6) wherein all of the X¹ to X³ are oxygen (one or two of X¹, X² and X³ may be a single bond or a (poly)oxyalkylene group) and metal salts of phosphorus compounds represented by formula (7) wherein all of the X⁴ to X⁷ are oxygen (one or two of X⁴, X⁵ and X⁶ may be a single bond or a (poly) oxyalkylene group). Component (B3) is preferably used because they can significantly enhance long-drain properties such as high temperature detergency, oxidation stability and base number retention properties.

The above-described metal salts of phosphorus compounds vary in structure depending on the valence of the metals or the number of OH group of the phosphorus compounds. Therefore, there is no particular restriction on the structure of the metal salts of phosphorus compounds. For example, when 1 mole of zinc oxide is reacted with 2 mole of a phosphoric acid monoester (with one OH group), it is assumed that a compound with a structure represented by formula (9) below be produced as the main component but polymerized molecules may also exist:

For another example, when 1 mole of zinc oxide is reacted with 1 mole of a phosphoric acid diester (with two OH groups), it is assumed that a compound with a structure represented by the formula below be produced as the main component but polymerized molecules may also exist:

Preferable examples of Component (B3) include salts of phosphorous acid diesters having two alkyl or aryl groups having 3 to 18 carbon atoms and zinc; salts of phosphoric acid monoesters having one alkyl or aryl group having 3 to 18 carbon atoms and zinc; salts of phosphoric acid diesters having two alkyl or aryl groups having 3 to 18 carbon atoms and zinc; and salts of phosphonic acid monoesters having two alkyl or alkenyl groups having 1 to 18 carbon atoms and zinc. These components may be used alone or in combination.

The phosphorus-containing compound contained in the lubricating oil composition of the present invention is a metal salt of a phosphorus-containing acid ester, the S/P (molar ratio) of which is preferably 1.5 or less, more preferably 1 or less, most preferably 0.

The content of the phosphorus-containing compound in the lubricating oil composition is preferably 0.2 percent by mass or less, more preferably 0.1 percent by mass or less in terms of phosphorus in view of high temperature detergency and base number retention properties. The content of the phosphorus-containing compound is preferably 0.005 percent by mass or more, more preferably 0.01 percent by mass or more, more preferably 0.02 percent by mass or more, particularly preferably 0.04 percent by mass or more in terms of phosphorus with the objective of easily inhibiting wear caused by soot contaminating in the composition.

Preferably, the lubricating oil composition of the present invention further comprises at least one type of additive selected from the group consisting of (C) metallic detergents, (D) ashless dispersants and (E) anti-oxidants.

Specific examples of (C) metallic detergents include sulfonate, phenate, and salicylate detergents.

There is no particular restriction on the structure of the sulfonate detergent. Examples of the sulfonate detergent include alkali metal or alkaline earth metal salts, particularly preferably magnesium and/or calcium salts, of alkyl aromatic sulfonic acids, obtained by sulfonating alkyl aromatic compounds having a molecular weight of 100 to 1,500, preferably 200 to 700. Specific examples of the alkyl aromatic sulfonic acids include petroleum sulfonic acids and synthetic sulfonic acids. The petroleum sulfonic acids may be those produced by sulfonating an alkyl aromatic compound contained in the lubricant fraction of a mineral oil or may be mahogany acid by-produced upon production of white oil. The synthetic sulfonic acids may be those produced by sulfonating an alkyl benzene having a straight-chain or branched alkyl group, produced as a by-product from a plant for producing an alkyl benzene used as the raw material of a detergent or produced by alkylating polyolefin to benzene, or those produced by sulfonating dinonylnaphthalene. There is no particular restriction on the sulfonating agent used for sulfonating these alkyl aromatic compounds. The sulfonating agent may be fuming sulfuric acids or sulfuric acid.

The sulfonate detergents include not only neutral alkaline earth metal sulfonates produced by reacting the above-mentioned alkyl aromatic sulfonic acid directly with an alkaline earth metal base such as an oxide or hydroxide of an alkaline earth metal such as magnesium and/or calcium or produced by once converting the alkyl aromatic sulfonic acid to an alkali metal salt such as a sodium salt or a potassium salt and then substituting the alkali metal salt with an alkaline earth metal salt; but also basic alkaline earth metal sulfonates produced by heating such neutral alkaline earth metal salts and an excess amount of an alkaline earth metal salt or an alkaline earth metal base (hydroxide or oxide) in the presence of water; and carbonate overbased alkaline earth metal sulfonates and borate overbased alkaline earth metal sulfonates produced by reacting such neutral alkaline earth metal sulfonates with an alkaline earth metal base in the presence of carbonic acid gas and/or boric acid or borate.

The sulfonate detergent used in the present invention may be any of the above-described neutral, basic and overbased alkaline earth metal sulfonates and mixtures thereof.

There is no particular restriction on the structure of the salicylate detergent. However, the salicylate detergent is preferably a metal salt, preferably alkali metal or alkaline earth metal salt, particularly preferably magnesium and/or calcium salt of an salicylic acid having one or two alkyl groups having 1 to 40 carbon atoms.

The salicylate detergent used in the present invention is preferably that the component ratio of which monoalkylsalicylic acid metal salt is higher because of its excellent low temperature viscosity characteristics and thus for example is preferably an alkylsalicylic acid metal salt and/or an (overbased) basic salt thereof, the component ratios of which monoalkylsalicylic acid metal salt and dialkylsalicylic acid metal salt are from 85 to 100 percent by mole and from 0 to 15 percent by mole respectively, and the component ratio of which 3-alkylsalicylic acid metal salt is from 40 to 100 percent by mole. The salicylate detergent is preferably that containing a dialkyl salicylic acid metal salt because of its excellent high temperature detergency and base number retention properties.

Examples of the alkyl group of the alkylsalicylic acid metal salt constituting the salicylate detergent include alkyl groups having 10 to 40, preferably 10 to 19 or 20 to 30, more preferably 14 to 18 or 20 to 26, particularly preferably 14 to 18 carbon atoms. Examples of the alkyl groups having 10 to 40 carbon atoms include decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl, eicosyl, heneicosyl, docosyl, tricosyl, tetracosyl, pentacosyl, hexacosyl, heptacosyl, octacosyl, nonacosyl, and triacontyl groups. These alkyl groups may be straight-chain or branched and primary, secondary or tertiary alkyl groups. However, secondary alkyl groups are preferable with the objective of easily producing the above-described desired salicylic acid metal salt.

Examples of the metal of the alkylsalicylic acid metal salt include alkali metals such as sodium and potassium, and alkaline earth metals such as calcium and magnesium. The metal is preferably calcium or magnesium, particularly preferably calcium.

The salicylate detergent used in the present invention also includes basic salts produced by heating an alkali metal or alkaline earth metal salicylate (neutral salt) produced as described above, and an excess amount of an alkali metal or alkaline earth metal salt or an alkali metal or alkaline earth metal base (hydroxide or oxide of an alkali metal or alkaline earth metal) in the presence of water; and overbased salts produced by reacting such a neutral salt with a base such as a hydroxide of an alkali metal or alkaline earth metal in the presence of carbonic acid gas and/or boric acid or borate.

Specific examples of the phenate detergent include alkaline earth metal salts, particularly magnesium salts and/or calcium salts, of an alkylphenolsulfide produced by reacting an alkylphenol having at least one straight-chain or branched alkyl group having 4 to 40, preferably 6 to 18 carbon atoms with sulfur or a Mannich reaction product of an alkylphenol obtained by reacting such an alkylphenol with formaldehyde.

The phenate detergent used in the present invention also includes basic salts produced by heating an alkali metal or alkaline earth metal phenate (neutral salt) produced as described above, and an excess amount of an alkali metal or alkaline earth metal salt or an alkali metal or alkaline earth metal base (hydroxide or oxide of an alkali metal or alkaline earth metal) in the presence of water; and overbased salts produced by reacting such a neutral salt with a base such as a hydroxide of an alkali metal or alkaline earth metal in the presence of carbonic acid gas and/or boric acid or borate.

The metallic detergent used in the present invention is preferably a detergent of an alkaline earth metal such as calcium or magnesium, particularly preferably a sulfonate detergent.

The base number of the metallic detergent used in the present invention is optional and usually from 0 to 500 mgKOH/g. However, it is preferable to use a metallic detergent with a base number preferably from 150 to 450 mgKOH/g, more preferably from 200 to 400 mgKOH/g in order to produce a lubricating oil composition which is excellent in base number retention properties and high temperature detergency and in particular anti-wear properties.

The term “base number” used herein denotes the base number measured by the perchloric acid potentiometric titration method in accordance with section 7 of JIS K2501 “Petroleum products and lubricants-Determination of neutralization number”.

Although metallic detergents are usually commercially available as diluted with a light lubricating base oil, it is preferable to use a metallic detergent whose metal content is from 1.0 to 20 percent by mass, preferably from 2.0 to 16 percent by mass.

Although the content of the metallic detergent in the lubricating oil composition of the present invention is arbitrary, the content is preferably 0.005 percent by mass or more, more preferably 0.05 percent by mass or more, more preferably 0.08 percent by mass or more and preferably 1 percent by mass or less, more preferably 0.5 percent by mass or less, more preferably 0.25 percent by mass or less, particularly preferably 0.15 percent by mass or less, all in terms of metal content. If the content is less than 0.005 percent by mass, the resulting composition would be likely deteriorated in high temperature detergency and anti-wear properties. If the content exceeds 1 percent by mass, the resulting composition facilitates the deterioration of exhaust-gas after-treatment devices such as DPF.

Component (D), ashless dispersant of the lubricating oil composition of the present invention may be any ashless dispersant that is usually used for a lubricating oil. Examples of the ashless dispersant include nitrogen-containing compounds having at least one straight-chain or branched alkyl or alkenyl group having 40 to 400 carbon atoms in their molecules and derivatives thereof, and modified products of alkenyl succinimides. A mixture of any one or more of these compounds may be blended with the lubricating oil composition of the present invention.

The carbon number of the alkyl or alkenyl group is from 40 to 400, preferably from 60 to 350. An alkyl or alkenyl group having fewer than 40 carbon atoms would deteriorate the solubility of the compound in a lubricating base oil, while an alkyl or alkenyl group having more than 400 carbon atoms would deteriorate the low temperature fluidity of the resulting lubricating oil composition. The alkyl or alkenyl group may be straight-chain or branched but is preferably a branched alkyl or alkenyl group derived from an oligomer of an olefin such as propylene, 1-butene, and isobutylene or from a cooligomer of ethylene and propylene.

There is no particular restriction on the nitrogen content of a nitrogen-containing compound that is an example of the ashless dispersant. However, the nitrogen content is from 0.01 to 10 percent by mass, preferably from 0.1 to 10 percent by mass in view of base number retention properties, high temperature detergency and anti-wear properties.

Specific examples of the ashless dispersant include the following compounds, one or more of which may be used:

(D1) succinimides having in their molecules at least one alkyl or alkenyl group having 40 to 400 carbon atoms and derivatives thereof; (D2) benzylamines having in their molecules at least one alkyl or alkenyl group having 40 to 400 carbon atoms and derivatives thereof; and (D3) polyamines having in their molecules at least one alkyl or alkenyl group having 40 to 400 carbon atoms and derivatives thereof.

Specific examples of (D1) succinimides include compounds represented by formulas (11) and (12):

wherein R²⁰ is an alkyl or alkenyl group having 40 to 400, preferably 60 to 350, and b is an integer of 1 to 5, preferably 2 to 4; and

wherein R²¹ and R²² are each independently an alkyl or alkenyl group having 40 to 400, preferably 60 to 350 carbon atoms, and particularly preferably a polybutenyl group, and c is an integer of 0 to 4, preferably 1 to 3.

Succinimides include mono-type succinimides wherein a succinic anhydride is added to one end of a polyamine, as represented by formula (11) and bis-type succinimides wherein a succinic anhydride is added to both ends of a polyamine, as represented by formula (12). The lubricating oil composition may contain either type of the succinimides or mixtures thereof.

There is no particular restriction on the method of producing these succinimides. For example, there may be used a method wherein an alkyl or alkenyl succinimide produced by reacting a compound having an alkyl or alkenyl group having 40 to 400 carbon atoms with maleic anhydride at a temperature of 100 to 200° C. is reacted with a polyamine. Examples of the polyamine include diethylene triamine, triethylene tetramine, tetraethylene pentamine, and pentaethylene hexamine.

Specific examples of (D2) benzylamines include compounds represented by formula (13):

wherein R²³ is an alkyl or alkenyl group having 40 to 400, preferably 60 to 350 carbon atoms, and d is an integer of 1 to 5, preferably 2 to 4.

There is no particular restriction on the method for producing the benzylamines. They may be produced by reacting a polyolefin such as a propylene oligomer, polybutene, or ethylene-α-olefin copolymer with a phenol so as to produce an alkylphenol and then subjecting the alkylphenol to Mannich reaction with formaldehyde and a polyamine such as diethylenetriamine, triethylenetetramine, tetraethylenepentamine, or pentaethylenehexamine.

Specific examples of (D3) polyamines include compounds represented by formula (14):

wherein R²⁴ is an alkyl or alkenyl group having 40 to 400, preferably 60 to 350, and e is an integer of 1 to 5, preferably 2 to 4.

There is no particular restriction on the method for producing the polyamines. For example, the polyamines may be produced by chlorinating a polyolefin such as a propylene oligomer, polybutene, or ethylene-α-olefin copolymer and reacting the chlorinated polyolefin with ammonia or a polyamine such as ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, and pentaethylenehexamine.

Specific examples of a derivative of a nitrogen-containing compound that is an example of (D) ashless dispersant include an organic acid-modified compound produced by allowing any of the above-described nitrogen-containing compounds to react with a monocarboxylic acid having 1 to 30 carbon atoms, such as fatty acid or a polycarboxylic acid having 2 to 30 carbon atoms, such as oxalic acid, phthalic acid, trimellitic acid, and pyromellitic acid, so as to neutralize or amidize the whole or part of the remaining amino and/or imino groups; a boron-modified compound produced by allowing any of the above-described nitrogen-containing compounds to react with boric acid so as to neutralize or amidize the whole or part of the remaining amino and/or imino groups; a phosphoric acid-modified compound produced by allowing any of the above-described nitrogen-containing compounds to react with phosphoric acid so as to neutralize or amidize the whole or part of the remaining amino and/or imino groups; a sulfur-modified compound produced by allowing any of the above-described nitrogen-containing compounds to react with a sulfuric compound; and a modified product produced by combining two or more selected from the modifications with an organic acid, boron, phosphoric acid, and sulfur, of the above-described nitrogen-containing compounds. Among these derivatives, boron-modified compounds of alkenylsuccinimides are excellent in heat resistance, anti-oxidation properties, and anti-wear properties and thus effective in further improving the base number retention properties of the resulting lubricating oil composition of the present invention.

When the lubricating oil composition of the present invention contains (D) ashless dispersant, there is no particular restriction on the content thereof. The content is preferably from 0.01 to 20 percent by mass, more preferably 0.1 to 10 percent by mass on the basis of the total mass of the composition. Component (D) of less than 0.01 percent by mass fails to enhance base number retention properties, high temperature detergency and anti-wear properties, while Component (D) of more than 20 percent by mass deteriorates extremely the low temperature fluidity of the resulting lubricating oil composition. When a boron-containing succinimide ashless dispersant is contained as Component (D), the resulting composition will be excellent in high temperature detergency. The content of the ashless dispersant is preferably from 0.001 to 0.1 percent by mass, more preferably from 0.005 to 0.04 percent by mass, more preferably from 0.01 to 0.02 percent by mass in terms of boron on the basis of the total mass of the composition.

Eligible Component (E), i.e., anti-oxidant include ashless anti-oxidants such as phenolic or aminic anti-oxidants and metallic anti-oxidants such as molybdenum anti-oxidants which are generally used for lubricating oil. Addition of an anti-oxidant can further enhance the anti-oxidation properties of a lubricating oil composition and thus can further enhance the base number retention properties thereof.

Specific preferable examples of the phenolic anti-oxidants include 4,4′-methylenebis(2,6-di-tert-butylphenol), 4,4′-bis(2,6-di-tert-butylphenol), 4,4′-bis(2-methyl-6-tert-butylphenol), 2,2′-methylenebis(4-ethyl-6-tert-butylphenol), 2,2′-methylenebis(4-methyl-6-tert-butylphenol), 4,4′-butylidenebis(3-methyl-6-tert-butylphenol), 4,4′-isopropylidenebis(2,6-di-tert-butylphenol), 2,2′-methylenebis(4-methyl-6-nonylphenol), 2,2′-isobutylidenebis(4,6-dimethylphenol), 2,2′-methylenebis(4-methyl-6-cyclohexylphenol), 2,6-di-tert-butyl-4-methylphenol, 2,6-di-tert-butyl-4-ethylphenol, 2,4-dimethyl-6-tert-butylphenol, 2,6-di-tert-α-dimethylamino-p-cresol, 2,6′-di-tert-butyl-4(N,N′-dimethylaminomethylphenol), 4,4′-thiobis(2-methyl-6-tert-butylphenol), 4,4′-thiobis(3-methyl-6-tert-butylphenol), 2,2′-thiobis(4-methyl-6-tert-butylphenol), bis(3-methyl-4-hydroxy-5-tert-butylbenzyl)sulfide, bis(3,5-di-tert-butyl-4-hydroxybenzyl)sulfide, 2,2′-thio-diethylenebis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], tridecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate, pentaerythrityl-tetraquis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], and octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate. Mixtures of two or more of these compounds may be used.

Examples of the aminic anti-oxidants include phenyl-α-naphtylamines, alkylphenyl-α-naphtylamines, and dialkyldiphenylamine. Mixtures of two or more of these compounds may be used.

Examples of the molybdenum anti-oxidants include organic molybdenum anti-oxidants such as alkylamine complexes of oxymolybdenum sulfide or oxymolybdenum, alkenylsuccinimide complexes of oxymolybdenum sulfide or oxymolybdenum, sulfurized oxymolybdenum dithiocarbamate and sulfurized oxymolybdenum dithiophosphate. One or more of these anti-oxidants may be blended. Among these anti-oxidants, it is particularly preferable to blend one or more types selected from oxymolybdenum sulfide- or oxymolybdenum-ditricecylamine complexes and oxymolybdenum sulfide- or oxymolybdenum-alkenylsuccinimide complexes because they can inhibit the viscosity of the resulting composition from increasing and thus can maintain fuel-saving properties for a long time and are excellent in high temperature detergency.

The above-described phenolic anti-oxidants, aminic anti-oxidants and molybdenum anti-oxidants may be blended in a suitable combination.

When (E) anti-oxidant is contained in the lubricating oil composition of the present invention, it is contained in an amount of 5 percent by mass or less, more preferably 3 percent by mass, more preferably 2 percent by mass or less, on the basis of the total mass of the composition. Component (E) of more than 5 percent by mass fails to obtain sufficient anti-oxidation properties as balanced with the content. Whereas, Component (E) is contained in an amount of 0.01 percent by mass or more, more preferably 0.1 percent by mass or more, more preferably 0.8 percent by mass or more, on the basis of the total mass of the composition with the objective of further enhancing base number retention properties and high temperature detergency.

When a molybdenum anti-oxidant is blended, the content thereof is from 0.001 to 0.2 percent by mass, preferably from 0.05 to 0.1 percent by mass, more preferably from 0.01 to 0.04 percent by mass, particularly preferably from 0.01 to 0.03 percent by mass in terms of molybdenum, on the basis of the total mass of the composition.

The lubricating oil composition of the present invention may be blended with any additives that have been conventionally used for lubricating oil, depending on their purposes, to an extent that the properties of the lubricating oil composition are not significantly deteriorated. Examples of such additives include anti-wear agents, friction modifiers, corrosion inhibitors, rust inhibitors, demulsifiers, metal deactivators, anti-foaming agents, and dyes.

The anti-wear agents may be any anti-wear agents that have been used for lubricating oil. For example, sulfuric extreme pressure additives may be used. Specific examples include dithiocarbamates, disulfides, polysulfides, sulfurized olefins, and sulfurized fats and oils.

There is no particular restriction on the content of these anti-wear agents if added. However, the content is usually from 0.01 to 5 percent by mass on the basis of the total mass of the composition.

Examples of the friction modifier include ashless friction modifiers such as amine compounds, fatty acid esters, fatty acid amides, fatty acids, aliphatic alcohols, aliphatic ethers and hydrazides (oleyl hydrazides), having at least one alkyl or alkenyl group having 6 to 30 carbon atoms, in particular straight-chain alkyl or alkenyl group having 6 to 30 carbon atoms per molecule, semicarbazide, urea (oley urea), ureide, and biuret, and metallic friction modifiers such as molybdenum dithiocarbamates and molybdenum dithiophosphates.

Examples of the corrosion inhibitors include benzotriazole-, tolyltriazole-, thiadiazole-, and imidazole-type compounds.

Examples of the rust inhibitor include petroleum sulfonates, alkylbenzene sulfonates, dinonylnaphthalene sulfonates, alkenyl succinic acid esters, polyhydric alcohol esters such as glycerin monooleate and sorbitan monoleate, and amines.

Examples of the demulsifiers include polyalkylene glycol-based non-ionic surfactants such as polyoxyethylenealkyl ethers, polyoxyethylenealkylphenyl ethers, and polyoxyethylenealkylnaphthyl ethers.

Examples of the metal deactivators include imidazolines, pyrimidine derivatives, alkylthiadiazoles, mercaptobenzothiazoles, benzotriazoles and derivatives thereof, 1,3,4-thiadiazolepolysulfide, 1,3,4-thiadiazolyl-2,5-bisdialkyldithiocarbamate, 2-(alkyldithio)benzoimidazole, and β-(o-carboxybenzylthio)propionitrile.

Examples of the anti-foaming agent include silicone, fluorosilicone, fluoroalkyl ethers.

When the lubricating oil composition of the present invention contains the above-described additives, the content of each of the friction modifier, corrosion inhibitor, rust inhibitor, and demulsifier is generally from 0.01 to 5 percent by mass, the content of the metal activator is generally from 0.005 to 1 percent by mass, and the content of the anti-foaming agent is generally from 0.0005 to 1 percent by mass, all on the basis of the total mass of the composition.

There is no particular restriction on the 100° C. kinematic viscosity of the lubricating oil composition of the present invention. However, the 100° C. kinematic viscosity is preferably from 4.0 to 27.0 mm²/s, more preferably from 5.6 to 16.3 mm²/s, more preferably from 9.3 to 12.5 mm²/s, particularly preferably from 9.3 to 11.0 mm²/s.

There is no particular restriction on the 40° C. kinematic viscosity of the lubricating oil composition of the present invention. However, the 40° C. kinematic viscosity is preferably from 14.0 to 200 mm²/s, more preferably from 20 to 110 mm²/s, more preferably from 40 to 60 mm²/s, particularly preferably from 45 to 55 mm²/s.

Preferably, the lubricating oil composition of the present invention has a 100° C. kinematic viscosity after shearing of 9.3 mm²/s or greater.

The viscosity grade of the lubricating oil composition of the present invention is preferably within the range of SAE20, 30, 40, and 50, more preferably within the range of SAE20 and 30, and particularly preferably within the range of SAE30, 0W-30, 5W-30, and 10W-30.

The viscosity index of the lubricating oil composition of the present invention is usually 160 or greater, preferably 170 or greater, more preferably 180 or greater with the objective of improving the viscosity-temperature characteristics and fuel-saving property and is preferably 250 or lower, more preferably 220 or lower, more preferably 200 or lower, particularly preferably 190 or lower in view of excellent shear stability, high temperature detergency and base number retention properties.

There is no particular restriction on the NOACK evaporation loss of the lubricating oil composition of the present invention. However, the NOACK evaporation loss is usually 20 percent by mass or less, preferably 16 percent by mass, more preferably from 10 to 15 percent by mass. A NOACk evaporation loss of greater than 20 percent by mass is not preferable because the evaporation loss amount of a lubricating oil increases.

The 150° C. TBS viscosity of the lubricating oil composition of the present invention is preferably 2.9 mPa·s or greater and lower than 3.7 mPa·s, more preferably 3.2 mPa·s or lower, more preferably 3 mPa·s or lower for SAE (xW-) 30 grade, for example, in view of the balance of a sufficient oil film formation capability and a friction reducing effect at a hydrodynamic lubrication area.

The term “TBS (Tapered bearing simulator) viscosity” used herein refers to an effective viscosity under high temperature shearing and is a viscosity at a shearing speed of 10⁶/s (provided that the temperature was set to 150° C. or 100° C.) measured in accordance with ASTM D4683 (Standard Test Method for Measuring Viscosity at High Shear Rate and High Temperature by Tapered Bearing Simulator).

The sulfated ash content of the lubricating oil composition of the present invention is preferably from 0.1 to 2 percent by mass, more preferably from 0.2 to 1 percent by mass, more preferably from 0.4 to 0.8 percent by mass.

The lubricating oil composition of the present invention is preferably used as a lubricating oil for internal combustion engines, such as gasoline engines, diesel engines and gas engines of motorcycles, automobiles, power generators, and ships and preferably used for automobiles, particularly preferably used for diesel engines thereof. Further, the lubricating oil composition is used as a lubricating oil that is required to have other properties such as anti-wear properties and long-drain properties, for example, as a lubricating oil for driving systems of automatic or manual transmissions, wet brake oils, hydraulic oils, turbine oils, compressor oils, bearing oils, refrigerating oils, or the like.

APPLICABILITY IN THE INDUSTRY

The lubricating oil composition of the present invention has a great industrial value as a lubricating oil for internal combustion engines because it can be reduced in lubrication resistance and thus further improve fuel-saving properties.

EXAMPLES

Hereinafter, the present invention will be described in more details by way of the following examples and comparative examples, which should not be construed as limiting the scope of the invention.

Examples 1 and 2, Comparative Examples 1 and 2

Asset forth in Table 1 below, various lubricating oils were prepared to measure the friction torque reduction rates thereof. The results are also set forth in Table 1.

(Friction Torque Reduction Rate Measurement)

The friction torque reduction rate measurement was carried out by measuring the friction torque of each oil under the following conditions using an engine motoring friction test.

Engine: 2 liter class in-line 4-cylinder roller type valve train engine

Engine speed: 750 to 3000 r/min

Oil temperatures: 60, 80, 100° C.

Reference oil: commercially available 5W30 oil

For evaluating the friction torque reduction rate, the friction torques measured under each of the above conditions are averaged out and compared with those of the reference oil to calculate the friction torque reduction rate.

As set forth in Table 1, it is confirmed that Example 1 wherein a base oil was blended with a styrene-diene copolymer viscosity index improver with a PSSI of 10 and Example 2 wherein a base oil was blended with a polymethacrylate viscosity index improver with a PSSI of 5 are improved in friction torque reduction rate with respect to the reference oil that is a commercially available 5W-30 oil. It is also confirmed that this effect is a substantial friction torque reduction rate, comparing with Comparative Examples 1 and 2 wherein a polymethacrylate viscosity index improver with a higher PSSI is blended on the basis of the conventional technical idea.

TABLE 1 Comparative Comparative Example 1 Example 2 Example 1 Example 2 Base oil A¹) balance balance balance Base oil B²) balance (A)Viscosity index improver A³) mass % 15 (A)Viscosity index improver B⁴) mass % 10 (A)Viscosity index improver C⁵) mass % 7 7 (B)Phosphorus anti-wear agent⁶) P amount, mass % 0.09 0.09 0.09 0.09 (C)Metallic detergent A⁷) Ca amount, mass % 0.1 0.1 0.1 0.1 (D)Ashless dispersant A⁸) mass % 6 6 6 6 B amount, mass % 0.01 0.01 0.01 0.01 (E)Anti-oxidant A⁹) mass % 1 1 1 1 (E)Anti-oxidant B¹⁰) mass % 0.15 0.15 0.15 0.15 Properties of composition Kinematic viscosity  40° C. mm²/s 52.7 45.7 52.9 57.1 100° C. mm²/s 9.91 9.5 11.6 12.3 Viscosity index 183 194 222 219 Kinematic viscosity after shear 100° C. mm²/s 9.7 9.3 9.3 9.5 Kinematic viscosity loss after shear % 2.1 2.1 19.8 22.8 TBS viscosity 100° C. mPa · s 6.14 6.60 6.64 6.89 TBS viscosity 150° C. mPa · s 2.9 3.1 3.2 3.3 Sulfated ash content mass % 0.55 0.55 0.55 0.55 NOACK evaporation loss amount mass % 12 12 12 13 (250° C., 1 h) Friction torque reduction rate (comparing 1.2% 0.8% 0.3% −0.4% with the reference oil) ¹)% C_(A) ^(:) 0, % C_(P)/% C_(N): 6.5, sulfur content: 0 mass ppm, 100° C. kinematic viscosity: 4.1 mm²/s, viscosity index: 123, 40° C. kinematic viscosity: 18.6 mm²/s, NOACK evaporation loss amount: 13 mass % ²)% C_(A) ^(:) 0, % C_(P)/% C_(N): 4.2, sulfur content: 0 mass ppm, 100° C. kinematic viscosity: 4.3 mm²/s, viscosity index: 123, 40° C. kinematic viscosity: 20.0 mm²/s, NOACK evaporation loss amount: 14 mass % ³)Styrene-diene copolymer viscosity index improver having polystyrene block-hydrogenated polyisoprene block, weight average molecular weight: 90,000, PSSI: 10 ⁴)Polymethacrylate viscosity index improver, weight average molecular weight: 100,000, PSSI: 5 ⁵)Polymethacrylate viscosity index improver, weight average molecular weight: 400,000, PSSI: 45 ⁶)Zinc di-n-butyl phosphate, phosphorus content: 13.2 mass %, sulfur content: 0 mass %, zinc content: 13.0 mass % ⁷)Overbased Ca sulfonate, base number (ASTM: D-2895): 325 mgKOH/g, Ca: 12.7 mass % S: 2 mass %, metal ratio: 10 ⁸)A mixture of polybutenyl succinimide (number average molecular weight of polybutenyl group: 1300) and boric acid-modified compound thereof ⁹)Phenolic- and aminic-anti-oxidants (1:1) ¹⁰)Ditridecylamine complex of oxymolybdenum, Mo: 10 mass % 

1. A lubricating oil composition comprising a lubricating base oil and a viscosity index improver with a PSSI of 30 or lower blended such that the viscosity index of the composition is 160 or greater and the 100° C. kinematic viscosity loss after shearing is 15 percent or less.
 2. The lubricating oil composition according to claim 1, wherein the viscosity index improver is a styrene-diene copolymer viscosity index improver with a PSSI of 30 or lower.
 3. The lubricating oil composition according to claim 1, wherein the viscosity index improver is a polymethacrylate viscosity index improver with a PSSI of 15 or lower.
 4. The lubricating oil composition according to claim 1, wherein the 100° C. TBS viscosity of the composition is 7 mPa·s or lower.
 5. The lubricating oil composition according to claim 1, wherein the 100° C. kinematic viscosity of the composition is from 9.3 to 11.0 mm²/s, the 40° C. kinematic viscosity is from 40 to 60 mm²/s and the 100° C. kinematic viscosity after shearing is 9.3 mm²/s or higher.
 6. The lubricating oil composition according to claim 1, further comprising a metal salt of a phosphorus-containing acid ester.
 7. The lubricating oil composition according claim 1, further comprising a metallic detergent, an ashless dispersant, and an anti-oxidant.
 8. The lubricating oil composition according to claim 1, wherein the sulfated ash content is from 0.1 to 2 percent by mass. 